The present invention relates in general to a transmission or recording of pulse code modulated signals, and in particular it relates to a pulse code modulation system in which a digital signal is passed through a recording- or transmission-channel, whereby each binary word of the digital signal is being correlated to an amplitude value of an analog signal.
In the case of a magnetic recording, but also during the transmission of digitally coded analog signals, the digital information carried by the signal may become partially altered due to interferences such as noise signals, for example. As a measure of the false information there is used the so-called bit error rate, which indicates the ratio of disturbed or false bits to the correctly reproduced ones. Values of the bit error rates occuring in practice lie in the range between 10.sup.-4 and 10.sup.-7. While the bit error rate by itself depends on the quality of the particular recording- or transmission channel, the effects of bit errors on the analog signal to be reproduced are dependent also on the kind of code employed. Accordingly, codes are desirable with which single bit errors exhibit a minimum interference effect on the reconverted analog signal. As a part of the interference effect is also meant the disturbance in perception (or a perception error) by eye or ear of a viewer or listener caused by an amplitude error in the reconverted analog signal due to a bit error in the code. For instance, experiments and theoretical considerations have unambiguously shown that for a certain degree of distortion in perception of a TV picture (a perception error) produced on a viewer by an amplitude error in a TV signal, a substantially higher amount of single bit errors in code words assigned to smaller amplitude levels is permissible than the number of single bit errors in code words assigned to higher amplitude levels of the PCM signal. It has been proved experimentally that in binary coded signals the bit error ate can be increased by a perception error factor of 10 in the case of an interference in less significant bits to obtain the same interference effect as in the case of a more significant bit differing by a single binary place.